NOTE: This spoiler was submitted by Jeremy

The film starts with Wihelmina “Billie” Logan (Brigette Lundy-Paine) and Thea “Theodora” Preston (Samara Weaving) telling the story of their dads, Bill S. Preston, Esq (Alex Winter) and Ted “Theodore” Logan (Keanu Reeves). They went through time and Hell to create great music as the Wyld Stallyns, but after achieving big success at one point, their music quality began to decline, and the world seemed to care less about them. Meanwhile, their actions have caused reality to bend, sending historical figures to different eras (Jesus switches places with Kid Cudi, for instance). To this day, they have not written the song that will unite the world.

In the present day, Bill and Ted are at a wedding to perform for Ted’s younger brother Deacon (Beck Bennett) and Missy (Amy Stoch), who married both Bill and Ted’s dads first. The guys play a mish-mash of instruments in their attempts to create their big song, but it just sounds bad, and Ted’s dad Jonathan (Hal Landon Jr.) thinks Bill and Ted are just washed up losers who waste their time and have been poor role models to their daughters, while also saying that their wives, Princess Joanna (Jayma Mays) and Princess Elizabeth (Erinn Hayes), are the only real adults in their families, but Billie and Thea still believe in their dads.

Liz and Jo bring their husbands to couples therapy, even though the intention was to do it separately. They talk to Dr. Wood (Jillian Bell), where it is apparent that Bill and Ted can’t do anything without each other, even tell their own wives “I love you”. Liz and Jo admit that while they love their husbands, they do feel that the two are just going through are a routine that they can’t grow out of.

Bill and Ted go home feeling upset over how they are viewed, with Ted even thinking of selling his Les Paul guitar to make money. Suddenly, an egg-shaped time machine appears. Kelly (Kristen School), daughter of the guys’ previous time-travel guide Rufus, tells Bill and Ted that they need to go with her. Billie and Thea watch as their dads hop in the time machine. The guys are taken 700 years into the future in San Dimas, where Kelly brings them to meet her mom, The Great Leader (Holland Taylor). She explains to the guys that since they still haven’t written the song that will unite the world, time and reality have begun bending in an event called “The Unraveling”, and if the guys can’t produce the song within less than 80 minutes (7:17 PM specifically), reality is doomed. Kelly gives them a pocket watch from her father that will keep track of time in San Dimas. Bill and Ted know they won’t have enough time, so they decide to try and go into their own futures to steal the song from themselves when they think they have already written it. They hop inside their old time traveling phone booth and head out.

The guys end up in 2022 to find their future selves are performing at a lounge for people who aren’t even listening to them. Present-Bill and Ted confront their future selves and learn that they have become bigger failures, not only in their music, but Liz and Jo left them after their own future selves showed them that there is no happy life with them, and their own daughters have stopped talking to them. The guys rush back to their present timeline to find their wives in therapy again, where they beg them not to join their future selves, but they still have a hard time saying “I love you” to them. Liz and Jo then see their future selves in their own time machine and head out. Dr. Wood also sees a historical figure appear on her couch, and she leaves.

Back in the future, the Great Leader tells Kelly that perhaps the only way to bring balance back to reality is to have Bill and Ted killed. She sends a time-traveling cyborg called Dennis (Anthony Carrigan) to hunt the duo. Kelly then uses her time machine to go back to present day San Dimas to find Bill and Ted, but instead finds Billie and Thea, and the two decide they want to help their dads unite the world. They use the machine to go back to 1968 to meet Jimi Hendrix (DazMann Still) and try and recruit him for their song by uniting some of the greatest artists in history. Jimi is uninterested, so the girls realize they’ll need bigger inspiration.

Bill and Ted go to 2025 where they come across a large mansion, thinking they have made it big as rock stars by now. They find their future selves dressed in flashy clothes and speaking with posh British accents that they say they got from time-traveling with their wives through the Medieval ages. They give their past selves what is supposedly the song that will unite the world for them to take back to their time, but as they are leaving, Present-Bill and Ted discover that the mansion belongs to Dave Grohl, and their old selves are actually balding, fat, alcoholics who are even worse than their 2022 selves. Jonathan and Deacon arrive with the SWAT team after Dave calls the cops, and since Future-Bill and Ted know what their past selves are going to do, Present-Bill and Ted come up with a crazy plan to cover their heads with buckets so their future selves won’t know where they went. The guys make it to their time machine, just as Dennis arrives and prepares to vaporize them, but he is too late, as the guys leave, and he accidentally zaps Jonathan. He ends up in Hell where he is greeted by Death (William Sadler).

Billie and Thea go to 1928 to find Louis Armstrong (Jeremiah Craft) where Billie shows him a video she recorded of Jimi onstage. Louis is impressed and goes to 1968 with the girls to meet Jimi. Although he still appears to not want to do it, especially since Louis Armstrong would be an old man at that time, Louis plays his trumpet, which is enough to convince Jimi to join them. They then take the two artists all the way back to Vienna in 1782 to find Wolfgang Amadeus Mozart (Daniel Dorr). They hear him performing one of his pieces, and Jimi takes out his guitar to mimic the notes. Mozart is impressed by what he hears and joins the team.

Bill and Ted then go to 2030 where they find their future selves in prison, both of them jacked and tattooed. They took the fall for breaking into Dave’s house, and now they plan to beat their past selves for it. However, Dennis shows up and attempts to kill Present-Bill and Ted, which Prison-Bill and Ted know will mean that they will die too. They get the inmates to pile on Dennis as Present-Bill and Ted make their escape.

Billie and Thea bring the three musicians to China in 2600 BCE to meet Ling Lun (Sharon Gee), the founder of music in China. Mozart plays her bamboo flute, and she joins them. The girls take them even further into the past to North Africa where they meet a dummer called Grom (Patty Anne Miller). They convince her to come along as well. With their band assembled, the girls return to present day San Dimas, where Kid Cudi also appears with time and reality still unraveling. He seems to understand the paradox that is occurring, just as Dennis shows up. He accidentally vaporizes everybody, as well as Kelly, and sends them all to Hell.

Bill and Ted decide to go all the way to the end of their lives, which is somewhere in 2067. They find themselves super old in a retirement home. They present their younger selves with a flash drive containing their big song, called “Face The Music” and labeled “Preston/Logan MP46 7:17” (MP46 is where they need to go). As they prepare to head back, Dennis shows up to get Bill and Ted, but his programming won’t allow him to kill them since they have the song. When they learn that he sent their daughters to Hell, Bill breaks the flash drive to get Dennis to kill them, but Dennis instead has a nervous breakdown and tries to kill himself. Bill and Ted jump into the path of the laser, which Liz and Jo see as their machine brings them to that time.

The guys end up back in Hell and go searching for their daughters. They find them, as well as Jonathan, who realizes that everything Bill and Ted told them about time travel and death was true. After reuniting, they figure that they can get back to Earth with Death’s help, but he had a falling out with Bill and Ted after they kicked him out of Wyld Stallyns for performing overlong solos. Billie and Thea talk to Death, as they are actually fans of his bass-playing, and they convince him to give their dads another chance. After Bill and Ted apologize, Death uses Jonathan’s SWAT van to bring the family, and the historical musicians, back up to Earth.

Everybody arrives in the present day on Interstate 210, which is where MP46 is. All of time and space starts coming undone, with monuments and other figures start jumping to other time periods and locations. Dennis gives Bill and Ted the pieces of the flash drive, but they still don’t know how to write their big song. They then realize that the “Preston/Logan” is not them, but rather Thea and Billie, as they are the masterminds behind the big song, and their dads are their to back them up. The girls give instruments to their band members, which doesn’t sound too great at first until they start to get things in sync. Liz and Jo show up and tell the guys that the happiest timeline with them is the one they are in now. Bill and Ted finally individually tell their wives they love them, and they all go through history to give everyone instruments to play, thereby actually uniting the whole world in song. When the guys return, they play epic guitar solos for the whole world to hear. Reality begins to set itself back to normal. Kelly and Dennis return to the future, where the Great Leader commends Kelly for her work, and everyone returns to their regular timelines. Billie and Thea end with their narration on how their dads helped unite the world by getting everyone to play together.

The credits show people all over the world playing music together. After the credits, we see Old Bill and Ted checking up on each other before standing up to do another epic guitar solo.

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Bill and Ted have yet to write the song that will unite the world, and they feel that they are going nowhere in life. Rufus's daughter Kelly appears from the future to bring them there, where Kelly's mom, the Great Leader, tells the guys that they must create the song by later that evening in their present time, or time and reality will collapse on itself. Bill and Ted then decide to time travel to when they have written the song so that they can save the world faster.

Their daughters, Billie and Thea, go on their own time-traveling adventure to help them get a band together by using famous historical musicians (Jimi Hendrix, Louis Armstrong, Mozart, Ling Lun, and a cavewoman drummer called Grom). However, the Great Leader thinks reality will be saved if Bill and Ted die, so she sends a cyborg named Dennis to hunt them down. In between their time-traveling, Bill and Ted learn that their wives, Princesses Joanna and Elizabeth, will leave them after they proved to be bad husbands, so they attempt to make sure they are successful in their music and their marriages.

After apparently securing the song from their elderly selves, Dennis finds Bill and Ted after having accidentally vaporized Billie, Thea, Kelly, the musicians, and Ted's dad, so the guys get Dennis to zap them so they can go to Hell to retrieve everyone. They have to apologize to Death, whom they had a falling out with after they kicked him out of their band. Billie and Thea convince Death to forgive their dads, and he helps them back to Earth.

As reality starts to come further undone, Bill and Ted realize it is Billie and Thea that created the big song. With help from Liz and Jo, they go through time to give everyone instruments so that the whole world will play the song together. Reality is fixed, everyone goes back to their timelines, and Bill and Ted are heroes in the eyes of their loved ones.